Process for producing zero-valent iron nanoparticles and treating acid mine drainage

ABSTRACT

A process for treating acid mine drainage removes iron ions from the acid mine drainage in the form of zero-valent iron nanoparticles which can be subsequently used for environmental remediation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationNo. 16/012,986 filed Jun. 20, 2018, and claims the benefit of thepriority of U.S. Provisional Application No. 62/618,880, filed Jan. 18,2018, which are incorporated by reference herein in their entireties.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to the treatment of acid mine drainage.

2. Brief Description of the Prior Art

Acid mine drainage is one of the most significant environmentalpollution problems associated with the mining industry. The main causeof this pollutant is the creation of pyrite and sulphide minerals aswater from rain and natural drainage moves through an underground mine.The mine's wastewater exposure to air upon discharge from the minecauses oxidation of the pyritic material which begins the leaching ofsulfuric acid and metals. The phenomena is called acid mine drainage(AMD) and it can be found flowing from underground mine tunnelsdischarging water as small highly contaminated streams feeding largerstreams during the process. Effluents are characterized by highconcentrations of heavy metals and high acidity, a combination that inmany cases causes severe environmental problems such as acidificationand lethal poisoning of aquatic organisms. Metal concentrations, whichcan be high, differ depending on the area of the country. Abandoned andworking mines continuously discharge mine drainage into surface andgroundwater bodies. Thus, the mining industry faces enormous challenges.

Acid mine drainage is a national problem, but one-third of watersimpacted by that problem are located in Pennsylvania, which, after overa century of coal extraction, has produced more coal tonnage than anyother state in the U.S. AMD is Pennsylvania's single largest non-pointsource water pollutant, impacting 2500 miles of streams (PA DEP, 1999b).AMD is formed when mining activities expose pyrite (iron disulfideminerals) to water and oxygen. Upon exposure to water and oxygen, pyriteoxidizes to form acidic drainage rich in dissolved metals.

Untreated AMD can severely degrade both habitat and water quality ofreceiving streams. This degradation is manifested by an alteration inthe macroinvertebrate community; specifically, there is a reduction inthe diversity and total numbers of macroinvertebrates and massive shiftsin community structure, favoring pollution tolerant species. In additionto the stress posed by a less abundant food source, fish are alsonegatively impacted by AMD directly. The primary causes of fish death inacid waters are loss of sodium ions from the blood and loss of oxygen inthe tissues. AMD contaminated groundwater can corrode and encrustman-made structures, causing serious problems. For example, AMD cancompromise well casings (water supply or oil and gas wells) which canlead to aquifer contamination. In the most severe cases, AMD renderswaters unfit for human use and recreation.

Contamination of groundwater with organic chemicals has been addressedin one approach by reducing the polluting organic using zero-valentmetals, such as iron. Nanoscale zero-valent iron is favored given thelarge surface area of zero-valent iron nanoparticles in comparison togranular iron.

In the past decade, the synthesis of iron nano particles has beenintensively developed not only for its fundamental scientificapplication, but also for many technological applications. For example,U.S. Pat. No. 7,674,526 discloses a process for preparing nanoscalezero-valent iron using reduction of a metal ion solution with adithionite compound which is disclosed to be advantageous in comparisonto a process using ferric ion and sodium borohydride as the reductant.

The chemical composition of acid mine discharge can vary significantlyfrom source to source, but often includes a variety of leached heavymetal ions, including ferric ion, ferrous ion, manganese ion, calciumion, magnesium ion, and aluminum containing ions, as well as silica.

There is a continuing need for methods of treating acid mine drainage toameliorate the environmental impact as well as a continuing need formaterials useful for treating environmental damage from harmful organiccompounds in ground .

SUMMARY OF THE INVENTION

The present invention provides a process for producing zero-valent ironand treating acid mine drainage. The process includes providing aqueousacid mine drainage feed stock including from 50 ppm to 500 ppm of metalion selected from the group consisting of ferrous iron, ferric iron, andmixtures thereof at a pH of less than 6.9, preferably from 3.5 to 6.9.Preferably, the acid mine drainage feedstock is provided at a pH of fromabout 6.1 to 6.8. More preferably, the acid mine drainage feedstock isprovided at a pH of from about 6.4 to 6.6.

The process further includes providing an alkali metal borohydrideselected from the group consisting of sodium borohydride, potassiumborohydride and mixtures thereof. In addition, the process furthercomprises mixing the alkali metal borohydride with the acid minedrainage feed stock to form an aqueous suspension of zero-valent iron.Preferably, the amount of alkali metal borohydride mixed with the acidmine drainage feedstock is sufficient to raise the pH of the aqueoussuspension to no more than 8.3. Preferably, the amount of alkali metalborohydride is sufficient to raise the pH of the aqueous suspension tofrom about 7.90 to about 8.15.

Preferably, the rate of addition of the alkali metal borohydride to theacid mine drainage is controlled to control the particle size of theresulting zero-valent iron.

Preferably, the acid mine drainage feedstock includes from about 100 ppmto about 400 ppm of metal ion. More preferably, the acid mine drainagefeedstock includes from about 200 ppm to about 300 ppm of metal ion. Itis preferred that the metal ion comprises at least 90 percent by weightferrous iron. Preferably, the process further comprises providing aninert atmosphere, and mixing the alkali metal borohydride with the acidmine drainage feedstock under an inert atmosphere. Preferably, theprocess further comprises separating the zero-valent iron from theaqueous suspension to provide separated zero-valent iron. In one aspect,the zero-valent iron is preferably separated by filtration. In anotheraspect, the zero-valent iron is preferably separated magnetically.Preferably, the process further comprises storing the separatedzero-valent iron in a medium having less than 5 ppm dissolved oxygen,and preferably less than 2 ppm dissolved oxygen. Preferably, the processfurther comprises storing the separated zero-valent iron in ethanol. Inone aspect, the alkali metal borohydride is mixed as a powder with theacid mine drainage feedstock. Preferably, the alkali metal borohyridepowder has a mean particle size of from about 20 to about 200 mesh.Preferably, the alkali metal hydride is mixed as an aqueous solutionwith the acid mine drainage feedstock.

Preferably, from about 0.5 to 0.8 g alkali metal hydride per gram ofiron ion is mixed with the acid mine drainage feedstock. Preferably, thealkali metal hydride is mixed with the acid mine drainage feedstock at arate of from about 500 g to about 600 g alkali metal hydride per gram ofiron ion per minute yielding 750 g to 1,000 g zero-valent iron. Inanother aspect, the effluent includes manganese ion, and the processfurther comprises further treating the effluent to precipitate maganeseion from the effluent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus for carrying out the processof the present invention. FIG. 2 is a HAADF-STEM image of a sample ofzero-valent iron produced by the process of the present invention.

FIG. 3 is a higher magnification TEM image of the sample of FIG. 2showing details of the internal structure.

FIG. 4 is a higher magnification HAADF-STEM image showing an oxide shellaround the zero-valent iron particles of the sample of FIG. 2.

FIG. 5 is a higher magnification HAADF-STEM/EDS image showing an oxideshell around the zero-valent iron particles of the sample of FIG. 4.

FIG. 6 is a schematic diagram showing a continuous implementation of theprocess of the present invention.

FIG. 7 is a schematic diagram showing an apparatus used in a presentlypreferred process according to the present invention.

FIG. 8 is a schematic diagram showing the removable magnetic filter usedin the apparatus of FIG. 7.

FIGS. 9 and 10 are a scanning electron micrograph of nano zero valentiron (“nZVI”) particles from produced by a process employing theapparatus of FIG. 7.

FIGS. 11 and 12 provide the results of energy-dispersive X-rayspectroscopy (“EDS”) mapping of a nZVI particle produced by a processemploying the apparatus of FIG. 7.

DETAILED DESCRIPTION

Nano scale materials that are typically in the range of 1-150 nm havedifferences in chemical, catalytic, electronic, magnetic, mechanical,and optical properties according to micro- or macro-scale materials.When the material comes up to nano sizes from micro sizes, theproportion of atoms located at the surface increases, thus theinteraction, adsorption, and reaction rates with other atoms, molecules,and complexes increase to achieve charge stabilization. Nanoparticlesare classified, according to their components, as organics, inorganics,metallic, semiconductors, ionics, and moleculars. The most notable amongthem are metallic nanoparticles because of their quantum size effectsand their large surface area. Many research articles published in thelast few years showed that nZVI has been successfully used for thetreatment of a wide variety of common environmental and chemicalcontaminants. For the degradation of aqueous organic solutes, both ironoxides and zero valent iron (“ZVI”) nanoparticles have been used as acatalyst The present invention provides a process for preparingzero-valent iron nanoparticles.

The process of the present invention can be carried out as a batchprocess or as a continuous process. The process permits the separationof dissolved iron from acid mine drainage. Dissolved iron is a majorconstituent of acid mine drainage. In addition to the dissolved iron,acid mine drainage frequently includes significant concentrations ofother heavy metals, such as manganese. The present process permitsdissolved iron to be separated from the acid mine drainage in the formof nano zero-valent iron, which can be subsequently employed forenvironmental remediation purposes. After removal of the zero-valentiron, the acid mine drainage can be subsequently treated to precipitateother heavy metals such as manganese which may be present and remove theheavy metal precipitate(s) from the acid mind drainage. Preferably,since the chemical properties of acid mine drainage can varysignificantly depending on the source of the acid mine drainage, thechemical properties, including the pH and the concentration of ferrousand ferric ions, manganese ion, et al., are measured before initiatingthe treatment process. Preferably, the concentration of iron ions in theacid mine drainage is at least about 50 ppm, more preferably from about50 ppm to about 500 ppm, still more preferably from about 100 ppm toabout 400 ppm, and still more preferably from about 200 to 300 ppm.While either acid mine drainage containing either ferric or ferrous ionsor a mixture of ferric and ferrous ions can be employed in the presentprocess, acid mine drainage including iron in the lower oxidation stateis preferred since the amount of reducing agent required to form thezero-valent iron and corresponding operation costs are correspondinglyreduced. Sodium borohydride, potassium borohydride, or mixtures of thetwo can be employed as alkali metal borohydride reducing agents,however, sodium borohydride is presently preferred. The alkali metalborohydride can be in the form of a powder or granular material, or inthe form of an aqueous solution, such as an aqueous solution of sodiumborohydride and sodium hydroxide available from Montgomery Chemicals,Conshohocken, Pa. When a sodium borohydride powder is employed, theparticle size and corresponding surface area of the powder can affectthe rate at which the powder is mixed with the acid mine discharge.Similarly, when an aqueous solution of sodium borohydride is employedthe concentration of sodium borohydride may affect the rate at which theaqueous solution is mixed with the acid mine discharge.

Zero-valent iron is susceptible to oxidation from dissolved oxygen inthe acid mine drainage. To minimize the extent of oxidation, the acidmine drainage is preferably purged with an inert gas such as nitrogen orargon prior to reducing the iron ions to metallic zero-valent iron withthe alkali metal borohydride. Preferably, the reduction reaction takesplace in a reactor vessel isolated from the atmosphere and/orcontinuously purged with an inert gas to minimize the level of dissolvedoxygen in the reaction mixture.

The reduction reaction of ferrous/ferric ion with borohydride ion hasbeen found to be sufficiently rapid such that reduction of the iron willtake place below a pH of about 8, and faster than the rapid hydrolysisof the borohydride in the acid mine drainage. Thus, zero-valent iron isformed, and the formation of insoluble iron and/or manganese oxide,hydroxide or carbonate is avoided.

The particle size of the zero-valent iron is a function of the rate ofaddition of sodium borohydride. For example, when sodium borohydride isadded at a slow rate, such as over an interval of 10 to 15 minutes, thepH of the acid mine drainage increases slowly, and the particle size ofthe resulting zero-valent iron is in the range of 50 to 200 nm. Incontrast, when the sodium borohydride is added at a faster rate, such asover an interval of 2 to 5 minutes, then the pH increases rapidly, andparticle size of the resulting zero-valent iron is in the range of 100to 300 nm. Thus, the mean particle size of the zero-valent iron can becontrolled by controlling the rate of addition of the sodiumborohydride.

Optionally, the particle size of the zero-valent iron can be controlledby addition of a suitable polymeric thickener material such ascarboxymethylcellulose.

It is presently understood that the rate of hydrolysis is given by:

Hydrolysis rate=k _(hyd)[H⁺][BH₄ ⁻]

And the reduction rate is given by:

Reduction rate=k _(Fe) ([Fe²⁺]/[H⁺]) [BH₄ ⁻]

Thus, there are two effective first-order rate constants (with respectto borohydride), k_(hyd) [H⁺] and k_(Fe) [Fe²⁺]/[H⁺], or k_(w) andk_(red) respectively. k_(w) will decrease with pH and k_(red) willincrease with pH. If k_(w) is 10 times faster than with k_(red) for thegiven iron content, even at pH 7.5, then more borohydride is required tospeed the reaction with the iron. The reason is simple: if there are twoparallel reactions, B and C involving the same reactant, A (i.e. BH₄ ⁻in this case), then:

d[B]/dt=−k ₁[A] d[C]/dt=−k ₂[A] d[A]/dt=−(k ₁ +k ₂)[A]

So

[A]=[A]₀ e ^(−(k1+k2)t) and [C]=[C]₀−(k ₂/(k ₁ +k ₂))[A]₀ (1−e^(−(k1+k2)t))

Therefore, the slow C reactant (assuming B is the fast one) will onlyuse up a fraction (k₂/(k₁+k₂)) of reactant A. As a result, if reductionis 10 times slower than hydrolysis, 10 times the borohydride is requiredto reduce all of the iron. Similarly, if reduction is 100 times slower,then we need 100 times higher borohydride concentration to reduce all ofthe ferrous iron.

The process of the present invention can be carried out in a batch modeor as a continuous process. An apparatus for carrying out the process inbatch mode is shown in FIG. 1. Untreated acid mine discharge 12 ispassed through a prefilter 14 to remove particulate matter (e.g. largerthan 20 microns) and deliver to a closed reactor 16 in which the acidmine discharge is continuously mixed by a mixing device 18. Nitrogen iscontinuously delivered to the reactor 16 from a tank 20 to purge oxygencontaining air from interior of the reactor 16 and especially from thereaction mixture 22 in the reactor 16. Sodium borohydride is added tothe reaction mixture through an injection port 24, and the pH andelectrode potential of the reaction mixture 22 is monitored using a pHmeter 26 as the sodium borohydride is added. When the reaction mixtureattains a preselected pH, the reaction mixture 22 is withdrawn from thereactor 16 by suitable means, such as a pump (not shown), and passedthrough an electromagnetic filter 28 which separates zero-valent iron 32from the reaction mixture 22 to produce a treated acid mine discharge30. The zero-valent iron 32 is subsequently removed from theelectromagnetic filter 28 and stored in ethanol to prevent oxidation.

EXAMPLE 1

A sample of acid mine drainage (“AMD”) was obtained from the Clyde minesite in Fredericktown, Pa. One liter of the AMD sample was filteredusing a 20μ cartridge filter. The pH of the AMD sample was 6.49, and thesample contained 227.6 ppm iron, 6.83 ppm manganese, and 0.87 ppmdissolved oxygen. The AMD water sample was purged with nitrogen whilemixing in the closed system shown schematically in FIG. 1. After about 3to 5 minutes, 0.01 g aliquots of solid sodium borohydride powderreducing agent were added to the AMD sample through a chemical injectorport and mixed, and after mixing for about one minute for each aliquot,the pH of the resulting mixture was measured, as reported in Table Abelow, along with electrochemical potential of the mixture, and a sampleof the reaction mixture was withdrawn from the reactor and visuallyinspected. Addition of the 0.01 aliquots of the sodium borohydridepowder continued until the pH of the resulting mixture was measured as8.05.

Hydrogen evolution from the mixture was apparent when the pH was 6.6.The resulting mixture as stirred for an additional 5 to 7 minutes andthen passed through a magnetic filter to collect black zero-valent ironparticles. After collection of the zero-valent iron particles, theresulting mixture was measured to have a pH of 8.13, 1.80 ppm iron, 4.77manganese, and 1.34 ppm dissolved oxygen.

The zero-valent iron produced was subjected to chemical and physicalanalysis at the Penn State Materials Research Institute. Samples wereprepared by sonicating the sample to disperse particles in solution. Aneedle was inserted into a rubber stopper to extract a small amount ofthe solution. The solution was drop cast on a lacey carbon transmissionelectron microscope (TEM) support grid and immediately inserted into theTEM under vacuum. The sample was exposed to atmosphere for less thanfive minutes. TEM and scanning transmission electron microscopy (“STEM”)were carried out using a Talos TEM at 200 kV (ThermoFisher Scientific)Energy-dispersive X-ray spectroscopy (“EDS”) mapping was carried out toinvestigate the composition of particles. High angle annular dark fieldSTEM (“HAADF”) providing a better mass contrast than TEM was alsocarried out, as the contrast was approximately proportional to Z², andreversed as compared to TEM.

FIGS. 2-5 are electron micrographs showing results of the analyses ofthe zero-valent iron produced by the present process. The images showlarge round elemental Fe particles approximately 100-300 nm in diameterwith a thin oxide shell (˜5 nm thick). Other elements present includeCa, Na, Mg, K, S. The results of EDS quantification of the elementalcomposition of a sample of the zero-valent iron particles are reportedin Table B. With the exception of Fe, O, Ca, most other elements seen inspectrum are at very low levels. Quantification shows four differentcompositions corresponding most likely to elemental iron, iron oxide,silicon oxide, and calcium oxide.

TABLE A Sodium borohydride (g) pH mV Notes 0 6.49 7.8 H₂ gas evolves0.01 6.51 1 Yellow/green magnetic ppt. 0.02 6.53 −3 Magnetic ppt. 0.036.6 −5.6 Dark green magnetic ppt. 0.04 6.72 −9.1 Magnetic ppt. 0.05 6.78−13.4 Black, magnetic ppt. 0.06 6.86 −16.3 Magnetic ppt. 0.07 6.93 −19.8Magnetic ppt. 0.08 7.02 −25 0.09 7.11 −29.8 0.1 7.25 −36.3 0.11 7.32 −420.12 7.5 −50 0.13 7.58 −55.3 0.14 7.62 −60.5 0.15 7.75 −65.5 0.16 7.83−71.3 0.17 7.94 −77 0.18 8.05 −80

TABLE B Error Mass C. Norm. C. Atom C. (3 sigma) Element Series Net (wt.%) (wt. %) (at. %) (wt %) S K series 527 0.42 0.42 0.57 0.14 Fe K series79988 83.82 83.82 66.05 7.68 O K series 11101 9.25 9.25 25.44 0.95 C Kseries 0 0 0 0 Na K series 890 0.60 0.60 1.14 0.16 Mg K series 535 0.390.39 0.71 0.14 K K series 1078 0.88 0.88 0.99 0.19 Ca K series 5366 4.654.65 5.11 0.54

It was found that at low pH-reaction yield was low and some Group I andGroup II chemical particle contamination was observed. At pH 8.3, higheriron nano crystals yield was observed as well as Group I and Group IIchemical particles contamination was observed. The nano crystals formedin the process were highly magnetic. No radioactive materials werecontained in the samples according to lab results. During the reactionnano crystals particle size could be controlled in the range 50-300 nmby controlling the rate of addition of the borohydride.

EXAMPLE 2

The process of Example 1 was repeated, except that an aqueous solutionof 10 percent by weight sodium borohydride/4 percent by weight sodiumhydroxide was prepared from a solution of 12 percent sodium borohydrideand 40 percent sodium hydroxide (BoroSpec™ 1240, Montgomery Chemicals,Conshohocken, Pa.) which was diluted 1:10 with distilled water, to whichwas added 8.8 g sodium borohydride dissolved in 100 ml water to form areducing agent. After purging a one liter sample of the AMD withnitrogen for around 3 to 5 minutes, 100 microliter aliquots of thereducing agent were added to the reaction mixture in the reactor andstirred, and samples were withdrawn for visual inspection, until themeasured pH of the reaction mixture was 8.15. The reaction mixture wasstirred for an additional 5 to 7 minutes and the treated reactionmixture was then passed through a filter to collect black zero-valentiron particles. Results are reported in Table C.

TABLE C Sodium borohydride (μL) pH Notes   0 6.49 M*  100 6.55 M  2006.7 M  300 6.77 M  400 6.88 M  500 7.13 M  600 7.25 M  700 7.34 M  8007.44 M  900 7.48 M 1000 7.56 1100 7.72 1200 7.8 1300 7.88 1400 7.95 15008.05 1600 8.19 *Indicates back magnetic iron precipitate.

EXAMPLE 3

A continuous implementation 100 of the process of the present inventionis shown schematically in FIG. 3. Prefiltered acid mine drainage 110 issupplied at about 1000 gallons per minute. The AMD has a pH of 6.48 andincludes about 200 to 250 ppm iron ions, and is supplied through volumecontrol solenoids 1121 a, 112 b, 112 c, to nitrogen purged treatmenttanks 116 a, 116 b, 116 c. Sodium borohydride is added through injectionports 114 a, 114 b, 114 c and the reaction mixture in each tank 116 a,116 b, 116 c is stirred to reduce the iron ions to zero-valent iron. ThepH in each tank 116 a, 116 b, 116 c is monitored, and when the pH isabout 8,1, the contents of the tanks is withdrawn by a pump 120 throughpH control valves 118 a, 118 b, 118 c and the reaction mixture is flowedthrough a magnetic filter unit 122. The treated acid mine waste 134 isstored in treated wastewater storage tank 132 for subsequent treatmentor disposal 142. The zero-valent ion 126 is washed from the magneticfilter 122 with ethanol and the resulting effluent is treated in adrying device 128 and the ethanol is recycled while the zero-valent ironis sent to a packaging device 130 to provide a packaged materialisolated from atmospheric oxygen.

EXAMPLE 4

Nano-size iron particles were synthesized using a borohydride reductionof ferrous iron present in Acid Mine Drainage (“AMD”). Magneticfiltration has been found to be one of the effective methods forseparation in a solution containing magnetic nano zero iron particles.Ferro magnetic materials that are magnetized by external magnetic fields(electro-magnets) are used by themselves or in conjunction with rareearth magnets as filtration elements in magnetic filters. The influenceof filtration operating conditions and particle diameter greatly impactthe behavior, and magnetic captures efficiency of the system underinvestigation. Magnetic nanoparticles were synthesized and capturedusing an array of neodymium magnets with an emphasis on manipulatingparticle size. Each sample of synthesized iron particles isolatedthrough magnetic filtration was characterized by transmission electronmicroscope (“TEM”) and wet chemistry techniques. The effect of synthesisparameters on the particle size of zero valent iron particles such as,initial ferrous iron concentration, borohydride concentration,temperature of the reaction, pH gradient as a function NaBH₄ additionrate, and liquid velocity were examined in detail. The results showinitial ferrous iron concentration relative to borohydride addition andthe rate of addition is the primary driver in the manipulation of nZVIparticle size.

High to moderate temperatures showed minimal to no effect on theproduction of particles in the range of 50-200 nm. The synthesizedparticles were determined to be mostly spherical at approximately 97%with representative single particle size given at 100-150 nm.

The synthesis and isolation of nZVI particles was carried out in a batchreactor by the reduction of ferrous iron in AMD water. The method ofreduction was reaction with Borohydride powder while applying an inertatmosphere of N₂ gas. NaBH₄ powder was used throughout the experiment.While the reaction of 4Fe²⁺+BH₄+3H₂O→4Fe(s)+H₃BO₃+7H⁺ to produce nZVI inthe lab is well known and characterized, the process according to thepresent invention used to produce and capture nZVI from anoxic AMD isshown FIG. 7.

A pilot testing procedure used for synthesis of nZVI and particleseparation is as follows:

Approximately 40 gallons of AMD water from the Clyde Mine site insouthwestern Pennsylvania containing ferrous ions was captured into aconical shape 50 gallon pilot tank. A nitrogen blanket was maintained inthe reaction vessel throughout the process. The system was started and are-circulation loop was established. This was done to ensure vigorousmixing (»400 rpm, to control particle size) of the NaBH₄. The amount ofNaBH₄ added was calculated based on concentration of iron at the time ofreaction (see FIG. 7). The reaction continued to completion based on afinal pH of approximately 8.3. The solution turned black, once complete;and the pilot tank re-circulation loop was discontinued andre-established through the magnetic filter. The velocity of the fluidwas then decreased in order to maximize the capture of particles throughthe magnetic filter. Due to the direction of flow, a gradient of nZVIparticles was formed on the magnets, the density of particles greatestfrom the lowest magnets to the highest in terms of the magnetic assemblyas illustrated in FIG. 8.

The reaction vessel transitions from black to visibly clear, as materialis removed from solution. At this stage, the magnet tree is rotatedthrough the wash station, extricating the product into a containmentvessel. The reaction vessel could now be emptied and ready to performanother batch.

Nano scale zero valent iron particles (nZVI) were produced using AMDwater from the Clyde site, and at the same time, total iron andmanganese concentration in the discharge stream were reduced well belowboth current Clyde and EPA water discharge limits as given in Table D.

TABLE D After current Clyde site Discharged peroxide water afterSelected (H₂O₂) nZVI Water Quality treatment production Parameter (ppm)(ppm) Total iron (Fe) 0.70 ± 0.05 0.20 ± 0.05 Total manganese (Mn) 2.10± 0.10 0.08 ± 0.01

Low boron (B) is considered as an essential element for plant growth anddevelopment

During the synthesis of nZVI, nanoparticles sizes were affected by thetemperature during the reduction process. The natural reaction medium ofnano-particle suspension is about 12° C., and the lowest particle sizeobtained at this temperature was around 50 nm. However, addition ofsodium borohydride and initial ferrous iron concentration in the AMD,are more effective parameters on the particle size than temperature.since the reaction occurs at all temperature conditions.

Isolated nZVI particles from magnetic filter were characterized by TEM,STEM for particle size. Large round elemental Fe particles approximately100 nm in diameter with thin oxide shell are found on SEM images asgiven in FIGS. 9 and 10.

The composition of nZVI particles were analyzed by EDS mapping whichconfirmed the purity is above 97%. In order to avoid further oxidation,the iron nanoparticle samples were prepared by drying them in a smallnitrogen-purged hood at room temperature and then packing them directlyin a sample cell. According to EDS, mapping other elements include Ca,C, Mg, Mn, are given in FIGS. 11 and 12. In order to maximize the valueof nZVI, the amount of Ca, Mg present as trace contaminants should beminimized.

Samples were prepared by sonicating sample to disperse particles insolution. A needle was inserted into rubber stopper to extract a smallamount of the solution. The solution was drop cast on a lacey carbon TEMsupport grid and immediately inserted into the TEM under vacuum. Thesample was exposed to atmosphere for less than 5 minutes. TEM and STEMdone on Talos TEM at 200 kV. EDS mapping was done to look at thecomposition of particles. HAADF-STEM—high angle annular darkfield—provided better mass contrast than TEM. The contrast wasapproximately proportional to Z2. Contrast was reversed as compared toTEM.

Various modifications can be made in the details of the variousembodiments of the articles of the present invention, all within thescope and spirit of the invention and defined by the appended claims.

1. Process for producing zero-valent iron and treating acid minedrainage, the process comprising: a) providing aqueous acid minedrainage feedstock including from 50 ppm to 500 ppm of metal ionselected from the group consisting of ferrous iron, ferric iron, andmixtures thereof at a pH of less than 6.9. b) providing an alkali metalborohydride selected from the group consisting of sodium borohydride,potassium borohydride and mixtures thereof, c) mixing the alkali metalborohydride with the acid mine drainage feedstock to form an aqueoussuspension of zero-valent iron.
 2. Process according to claim 1 whereinthe amount of alkali metal borohydride mixed with the acid mine drainagefeedstock is sufficient to raise the pH of the aqueous suspension to nomore than 8.3.
 3. Process according to claim 2 wherein the amount ofalkali metal borohydride is sufficient to raise the pH of the aqueoussuspension to from about 7.90 to about 8.15.
 4. Process according toclaim 1 wherein the acid mine drainage feedstock includes from about 100ppm to about 400 ppm of metal ion.
 5. Process according to claim 4wherein the acid mine drainage feedstock includes from about 200 ppm toabout 300 ppm of metal ion.
 6. Process according to claim 1 wherein themetal ion comprises at least 90 percent by weight ferrous iron. 7.Process according claim 1 further comprising providing an inertatmosphere, and mixing the alkali metal borohydride with the acid minedrainage feedstock under an inert atmosphere.
 8. Process according toclaim 1 further comprising separating the zero valent iron from theaqueous suspension to provide separated zero-valent iron and aneffluent.
 9. Process according to claim 8 wherein the zero-valent ironis separated by filtration.
 10. Process according to claim 8 wherein thezero-valent iron is separated magnetically.
 11. Process according toclaim 8 further comprising storing the separated zero-valent iron in amedium having less than 5 ppm dissolved oxygen.
 12. Process according toclaim 8 further comprising storing the separated zero-valent iron inethanol.
 13. Process according to claim 1 wherein the alkali metalhydride is mixed as an aqueous solution with the acid mine drainagefeedstock.
 14. Process according to claim 8 wherein the effluentincludes manganese ion, and the process comprises further treating theeffluent to precipitate maganese ion from the effluent.
 15. Processaccording to claim 1 wherein the acid mine drainage feedstock isprovided at a pH of from about 6.1 to 6.8.
 16. Process according toclaim 15 wherein the acid mine drainage feedstock is provided at a pH offrom about 6.4 to 6.6.
 17. Process according to claim 1 wherein the rateof addition of the alkali metal borohydride to the acid mine drainage iscontrolled to control the particle size of the resulting zero-valentiron.